Note that the terms “ballast” and “converter” are used equivalently in the context of the specification, drawings and claims.
The invention especially relates to actively switched power factor control circuits (PFC circuits), preferably Totem Pole PFC circuits, for operating lighting means, such as e.g. one or more LED or OLED. Especially, the present invention relates to a ballast for lighting means with an actively switched power factor correction stage (PFC stage), which preferably is implemented in a Totem Pole boost topology. The invention also relates to a lighting device including such ballast.
For power-conversion circuits generally, including drivers and converters for lighting means, such as ballasts for fluorescents and high-pressure lamps or ballasts for LED-modules, it is required to limit the amount of harmonics in the current which is drawn from the voltage supply, because in many countries regulatory laws are effective in this respect. Therefore, a simple solution consisting of rectification means (bridge- or discrete rectifier diodes) plus a bulk-storage (e.g. an electrolytic capacitor) is not suited for most applications, as the harmonics in the current would exceed the benchmarks set by regulatory laws. As a solution thereto, many switching mode power supplies (SMPS) that are used in ballasts for lighting means operate in a mode, i.e. comprise a PFC circuit, in order to draw a dominantly sinusoidal current with a low harmonics content. Many different converter topologies (boost-, buck-, SEPIC-, flyback, etc.) exist that can be used for implementing a PFC circuit.
A preferred topology according to the present invention is the so-called “Totem Pole” boost topology, which offers some benefits, such as an inherent possibility to entirely avoid switching-loss on the switch-devices, thus allowing the usage of higher switching frequencies. Due to the higher switching frequencies it is possible to design a converter with smaller geometric dimensions.
Incorporation of US 2013/0257390 A1 by reference: For the description of a preferred Totem Pole PFC circuit, i.e. an actively switched PFC circuit implemented in a Totem Pole boost topology, according to the present invention the US patent application publication US 2013/0257390 A1 is incorporated herewith by reference.
Definition of “Totem Pole PFC topology”: The term “Totem Pole PFC topology” as used herein refers to the PFC topology disclosed in the US patent application publication US 2013/0257390A1 and is defined as following:
The circuit comprises an AC power supply, a first half bridge arm and a second half bridge arm.
The first half bridge arm includes a first and a second switch connected in series with each other.
A second terminal of the first switch is connected to a first terminal of the second switch, and coupled to a first end of the AC power via a first inductor.
The second bridge arm includes third and fourth switches connected in series with each other.
A second terminal of the third switch is connected to a first terminal of the fourth switch and a second end of the AC power.
The third and fourth switches operate in ON/OFF states by use of a control signal having an operation frequency consistent with that of the AC power.
The on-state resistance of the third and fourth switches is lower than that of the first or second switch, thereby reducing the on-state loss of the switch and improving the operation frequency of the circuit. Since the third and fourth switches operate at the (low) operation frequency of the voltage of the AC power (about 50-60 Hz), which is very low, the switching loss and the drive loss, which are directly proportional to the operation frequency, are very small.
However, there are some drawbacks that are currently limiting the usability of Totem Pole boost topologies, such as high control effort, larger component count compared to other topologies, no easily accessible way of sensing the current through the switch and protection of the components due to a floating potential of both switch nodes etc. Namely, the Totem Pole boost topology requires driving means for switches, which are on a floating potential and are mostly on a much higher potential than the control circuitry for controlling the switches of the Totem Pole boost topology. Driver solutions for such a topology are known to those skilled in the art as “high side drivers” and usually have the disadvantage of being more complex and demanding a higher component effort compared to driver solutions for driving switches connected to ground potential (gnd).